RFID tags are now widely used to mark, inventory and track various products. RFID tags generally transmit to a handheld or robotically controlled reader device a radio frequency (RF) signal that includes product information. RFID tags generally include an integrated circuit for storing and processing information, a transceiver for transmitting and receiving RF signals, and an antenna. Some RFID tags are active RFID tags and include their own battery power source. Passive RFID tags do not have their own power source and require receiving a power signal from a reader device to operate. For interrogating passive RFID tags, a reader generally transmits a continuous wave (CW) or modulated RF signal to a tag. The tag receives the signal, and responds by modulating the signal and then “backscattering” an information response signal to the reader. The reader device receives the response signal from the tag, and the response signal is demodulated, decoded and further processed.
The receiver of a reader device employs a directional device to separate the incoming backscatter from the outgoing CW energy; however, any energy reflected back from an antenna of the reader device is summed with incoming backscatter signals. Cabling and RF-reflective objects in front of an antenna can also cause such reflected energy. Increasing a bias point (also known as an operating point) of an RF power amplifier of a reader device will generally increase a return loss value and increase an operating range of the amplifier. A bias point is therefore generally preset to a fixed value to accommodate an estimated worst-case return loss value.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.